So with a diesel you have to be care of people chopping them out, running them hard, the returning it to stock and trading it it. Usually higher mileage ones are like that. Lots of good videos on YouTube at stuff to look at on used diesels.
As for the gas one, the Mega cab is nice, but it makes the truck essentially a crew Cab long bed. Which makes it difficult in any city setting.
Both would serve you well, there is a guy who has 380,000 miles on his 6.4 and he’s only had to replace minor parts. He also runs it hard or near max tow/payload a lot.
With the diesel you can lid more stuff in trailer and won’t have near the issue with mountains or high elevation, gas will have better payload but will struggle more up mountains and higher elevation.
What to look for would be a 3500 CC SB 6.7 with low miles. That IMO is a perfect truck. Will have close to 4500lbs if payload and 20k trailer weight
2018 2500 6.4 4x4 4.10 Amsoil SS 0w-40, Softopper
Black Rhino Armory 18x9 +12mm
Cooper St MAXX LT295/70R18
2016 Durango 5.7 AWD 3.07 Amsoil SS 4 Qts 5W-30 and 3 Qts 0W-40
1996 Nissan Altima 2.4 Amsoil SS 5W-30